January 29, 2003 - Author
note: I first used this title for a
paper presented at the 1998 Crop and Pest Management Short Course,
St. Paul, MN. The following paper was presented at a
recent meeting in St. Joeseph, MO.

Weed management generally
doesn’t generate much interest among the general public, but the topic of
glyphosate resistant weeds was recently featured on the front page of the Des
Moines Register and New York Times. By now most Midwest farmers have experience
with at least one herbicide resistant weed species, so resistance is not a new
problem. The question is whether glyphosate resistance should be treated
different than resistance to other herbicides. This talk will review where we
stand in terms of glyphosate resistance and potential impacts of this problem.

Current status The first documented case
of glyphosate resistance was reported in 1996 involving rigid ryegrasss in
Australia. The resistant biotype was obtained from an orchard near Orange, New
South Wales, Australia. The site had intensive selection pressure, with two or
three applications per year of glyphosate for 15 years. Roundup was used to
control weeds within rows of trees. Greenhouse research confirmed that this
population was 7 to 11 times more resistant to Roundup than susceptible
populations. Since the original report several additional glyphosate resistant
weed populations have been identified: rigid ryegrass in a wheat production
system in Australia and in California, Italian ryegrass in Chile, goosegrass in
Malaysia and horseweed (marestail) in the east, midwest and southeast U.S.
Resistance in goosegrass is due to an altered target site, whereas the mechanism
of resistance in the other weeds is currently under investigation.

The identification of glyphosate resistant
horseweed is the first case of a weed developing resistance in Roundup Ready
soybeans. The resistant biotype first appeared in Delaware in 2000 and since
has spread as far west as Indiana and has been identified in the Southeastern
U.S. where Roundup Ready cotton is grown. The first population originated in a
field in which glyphosate was the only herbicide used in Roundup Ready soybeans
in 1999 and 2000. Prior to 1999 glyphosate had been used infrequently as a
preplant burndown herbicide in no-tillage systems. The horseweed biotype has
exhibited 8 to 13 fold resistance to glyphosate.

There has been considerable
discussion whether waterhemp should be included on the list of glyphosate
resistant weeds. Waterhemp populations with individuals capable of surviving
‘normal’ use rates were identified in Iowa and Missouri the first year that
Roundup Ready soybeans were marketed. Research in the greenhouse and laboratory
has shown that the enhanced tolerance/resistance exhibited by these plants is
controlled genetically, rather than environment, coverage, and other management
practices. A student working for Mike Owen at Iowa State University has been
able to increase the tolerance in one of these problem populations by 3.5 fold
after two generations with recurrent selection (see
article).

In academic terms, the research at both Missouri
and Iowa could be interpreted to support that glyphosate resistant waterhemp is
indeed present in the Midwest. Both groups have identified waterhemp
populations that survive higher levels of glyphosate than most populations, and
they have documented that the trait can be passed on to new generations.
However, I believe there are a few important pieces missing, and in my mind
prevent the problem waterhemp populations from being classified as glyphosate
resistant.

First, the populations were
identified the first time glyphosate was used as a postemergence herbicide in
the field (i.e. the first year RR soybeans were marketed). Resistance is
defined as the ability of a plant to survive a dose of herbicide that was toxic
to the original population. This implies that the resistance is identified
after selection pressure has weeded out the susceptible individuals. Since the
problem was identified the first time glyphosate was used for general weed
control, I feel these populations fail to meet the criteria of being selected
from the original population. Second, I am not aware of these problem
populations increasing following continued use of glyphosate. I am not aware
of any farmers or dealers in Iowa who have given up on controlling waterhemp
with glyphosate. Higher rates of glyphosate are currently being used than when
RR soybeans were first introduced, and the percentage of RR soybean fields
treated with a preemergence herbicide has increased dramatically. Waterhemp may
be largely responsible for both of these occurrences in many fields. However,
growers are still relying on glyphosate to control waterhemp postemergence, and
I’m not aware of anyone who adds a diphenylether to glyphosate to control
waterhemp in RR soybeans. For these reasons, I do not think waterhemp should be
included in the list of weeds resistant to glyphosate. However, the research
has documented that within the waterhemp gene pool there is the potential for
resistance to develop, and it is something that should be watched closely.

Impact of glyphosate
resistant weedsThere has been a lot of talk lately about the potential impacts of
glyphosate resistant weeds. Some persons have described them as super weeds,
and there have even been inferences that the presence of glyphosate resistant
weeds could reduce the value of farmland. There is no question that the
development of glyphosate resistant weeds will increase the cost of weed
management for farmers, but the question is by how much. I think there are
several possible scenarios: some situations would have relatively little impact
whereas others would pose a major problem for farmers. Which scenario develops
depends upon the characteristics of the resistant weed, primarily the
effectiveness of alternative tactics on this species and how quickly the weed
spreads.

For most weed species we have
alternatives to glyphosate that are highly effective and provide good
flexibility in application timing. For these weeds a farmer could simply add
another herbicide to glyphosate to control the resistant species. In this
situation, the primary impact of the glyphosate resistance is the added cost of the
additional herbicide, otherwise the farmer could use the identical weed
management program used prior to the development of the resistant velvetleaf
population. Farmers who rotate Roundup Ready corn and Roundup Ready soybeans
already do this by using Select or a similar herbicide to control RR volunteer
corn in their beans.

The more costly scenario would
involve a weed for which the alternative herbicides have limited flexibility in
application timing. A weed species that requires postemergence applications to
be made before weeds reach a 4-inch height would have a major impact on weed
management systems. In this situation, the loss of application flexibility
would present a greater cost to many growers than the additional herbicide
expense. The continued growth in farm size increases the
importance of the application flexibility
provided by glyphosate.

Since the first report of glyphosate resistant rigid
ryegrass in 1996, four additional resistant species with this trait have been
identified. While not quite one new species per year, this rate of development
suggests that we will continue to see new resistant biotypes. Eventually one of
these weeds will appear in Iowa and surrounding states, and my guess is it will
happen sooner than later. However, the ability to survive glyphosate does not
create a ‘super weed’, and there is no reason to use scare tactics to try and
change farmers' perceptions and practices.

I believe it is makes good sense for farmers to
implement a long-term plan to reduce the selection
pressure placed on weeds by glyphosate. The simplest way to do this is to
avoid planting continuous Roundup Ready crops. Using additional modes of
actions with glyphosate provides
alternative selection pressures on certain weeds, and in some situations this
will reduce the likelihood of resistance. However, since we do not
know which weed is likely to develop resistance, it is
impossible to know whether the
alternative mode of action is reducing glyphosate selection pressure
on the appropriate speces. Thus, we believe an annual
rotation of herbicides should be the foundation of resistance
management.

With the manner that
glyphosate is being used in the Midwest, resistance is inevitable. When
resistance develops, we will need to control these biotypes with existing
herbicides – no new modes of action are coming down the pipeline in the
foreseeable future. The large number of alternative products for use in corn
and soybean will reduce the impact of glyphosate resistance, but there can be
significant costs associated with the problem.
The need for application flexibility in today's agriculture increases the cost
of glyphosate resistance compared to previous cases of resistance experienced by
Iowa farmers. Because of this, evaluating weed management programs
in terms of selection pressure placed on weeds should be an important component
of crop management planning.

Common chemical and
trade names are used in this publication. The use of trade names is for clarity by the
reader. Inclusion of a trade name does not imply endorsement of that particular brand of
herbicide and exclusion does not imply nonapproval.